1. Field of the Invention
The present invention relates to a gradation printer for recording, for example, an image with a shading pattern by using a thermal printer.
2. Description of the Prior Art
Gradation printers are used as a terminal appliance of a captain system, INS, etc to hard copy supplied data. Such gradation printers represent picture data as a shading pattern by means of a matrix of printing dots.
A conventional gradation recording system for controlling the density of the printing dots will be explained with reference to a gradation printer using a thermal head shown in FIGS. 4 and 5.
In such a gradation recording system, heat-sensitive paper fed at a constant speed is colored by heat sensitization of a thermal head, and the density of a printing dot is represented by varying the coloring area by controlling the period for which the thermal head is driven in correspondence with the density gradation. For example, it is assumed that the printing time for one line of the gradation printer is T, and that N density gradations are recorded. If the thermal head is driven for the entire period of the printing time T, the coloring area of the printing dots occupies the maximum area of the printing space, thereby producing the maximum density gradation. On the other hand, if the thermal head is not driven at all in this period, the coloring area becomes zero, thereby producing the minimum density gradation, namely, the color being equal to the ground color of the white paper. If the printing time T is divided into (N-1), and the divided printing time is t, the formula t=T/(n-1) holds. A part of the printing space is colored by driving the thermal head by a period 0, t, 2t, . . . , or (N-1)t, which is obtained by multiplying t by an integer in accordance with the density degradation. The reason why T is divided into (N-1) is that one of the N gradations is white which does not need to be colored.
In a conventional 8-density gradation recording system, one printing time is divided into seven periods from t1 to t7, as shown in FIG. 5, and driving data from D11 to D 18 are supplied to the thermal head in accordance with the respective density gradation when a signal ENB for driving the thermal head is turned on. Current is applied to color the heat-sensitive paper during the period in which these driving data are at a high level. Since the heat-sensitive paper is fed at a constant speed, the coloring portion having the area corresponding to the period in which current is applied to the thermal head is formed from the top of the printing space, as shown in FIG. 4.
In this way, the density of each printing dot is represented by the size of the coloring area, and the matrix of such printing dots record the density gradation of one image as a shading pattern.
However, in the above-described conventional density gradation recording system, since the coloring portion of any density gradation is formed from the top of the printing space when the signal ENB is turned on, the density represented by a printing dot is sometimes mistakenly recognized as a different gradation in the image. For example, as shown in FIG. 6, when printing dots 9, 10 and 11 are arranged in the direction in which the heat-sensitive paper is fed, and if the density gradations of the printing dots 9, 10 and 11 are 8, 2 and 8, respectively, the coloring position of the printing dot 10 is not located at the center of the printing dots 9 and 11 but is located in proximity to the printing dot 9. Therefore, the blank portion between the printing dots 10 and 11 becomes large, which causes mistaken recognition of the gradation of the image when it is observed as a whole.